The principle of wet combustion has been known for many years. Wet combustion of pulping black liquors with air or oxygen under pressure is a subject of numerous open literature publications and patent applications since late 1940's. Among them: U.S. Pat. No. 2,665,249 dated Jan. 5, 1954 and Canadian equivalent No. 622,663 dated June 27, 1961 as invented by F. J. Zimmerman, U.S. Pat. No. 2,774,666 dated Dec. 18, 1956 and its Canadian equivalent No. 555,460 dated Apr. 1, 1958 as invented by E. W. Schoeffel and D. M. Barton. There is also Canadian Pat. No. 962,012 as invented by Pradt et al, Morgan and Saul's "The Zimmerman Process in Soda Pulp Mill Recovery Systems" Appita Vol. 22, No. 3 (November 1968) and Othmer, "The Wet Air Oxidation (WAO) of Waste", Mechanical Engineering (December 1979) p. 30-37.
Basically the same process has been carried out under various conditions with respect to temperature, pressure and oxidizing gas.
Various types of reactors have been proposed, including for instance: pipeline (as invented by E. L. Cole in Canadian Pat. No. 1,010,164 dated 77/05/10), continuous autoclave (as invented by W. M. Fassell et al in Canadian Pat. No. 977,621 dated Nov. 11, 1975 and U.S. Pat. No. 3,870,631 dated Mar. 11, 1975) or system of batch autoclaves (as invented by D. F. Othmer in Canadian Pat. No. 1,087,129 dated 80/10/07).
Most of the patents consider a complete oxidation of organic material for inorganic salts recovery and/or waste solution safe disposal. Nevertheless in a number of patents an incomplete combustion to make and recover short chain organic molecules such as carboxylic acids anions (e.g. acetate) is discussed.
In Canadian Pat. No. 1,087,129 dated Oct. 7, 1980 Othmer discloses various aspects of the carrying out of such oxidation. It is also known in a general way that oxidation can be total or partial depending upon the reaction conditions, and that it involves various side reactions: For instance, in partial oxidation the complete oxidation of some reactants is obtained, together with little or no oxidation of some other reactants.
Soda black liquor is produced as a result of high yield pulping process using soda ash, that is Na.sub.2 CO.sub.3, as a main pulping chemical.
The dissolved solids in the soda black liquor include three major classes of components, namely:
1. Lignin fragments of anionic nature, (also known as alkali lignins) from large polymeric fragments down to phenolates, such as vanilate etc, constitute about a half of the dissolved solids weight. PA0 2. Close to 40% of the dissolved solids are accounted for by the short chain hydroxy and carboxy acid anions such as lactate, acetate, propionate, citrate, formate, etc. PA0 3. Sodium carbonate or bicarbonate, depending on pH constitutes from 0 to 10% of the liquor.
Normally the amount of sodium ions naturally occuring in the black liquor obtained by soda pulping process corresponds to less than 20% of the total solids weight. In order to efficiently recover a desired product mixture and avoid problem associated with separation and disposal of high molecular weight by-products, the destruction of first group components must be as complete as possible by the end of the oxidation process without increasing the rate of short chain component destruction. For example: an increase in temperature shortens the time for aromatic lignin fragments decomposition but at the same time increases the rate of short chain components destruction. A decrease in the process temperature would lead to a less extensive destruction of the high molecular weight components within practically acceptable retention time.
Prior art recommends only interruption of oxidation when acetate concentration reaches its maximum thereby creating the necessity of further lignin separation steps. The various equilibria among the three major classes of components in soda black liquor as described above create serious difficulties when one wishes to favour short chain components formations such as acetate and the like.